The Generalized Planner Fault Energies Of Al₃Sc And Al₃Mg, And The Effect Of Rare-earth Elements On The Stacking Faults Of Mg

As lightweight structural materials with a good combination of mechanical properties such as low density, the highest strength-to-weight-ratio, good stiffness, excellent electromagnetic shielding ability, especially the significant advantages of easy-recycling, Mg alloys and Al-Mg alloys have been applied extensively in the field of aerospace, microelectronics and automobile industries. However, the application of magnesium alloys in the modern industry currently is still limited because of the limited tensile strength, inferior ductility, easily burnt and limited creep resistance at elevated temperature. In order to further improve their properties, many experimental investigations have been performed. Experimental investigations showed that addition of alloying elements could significantly improve the mechanical properties of Mg alloys. However, research on RE magnesium alloys has so far been focused mainly on experiments, the theoretical study is very scarce, and there is little information about deformation behavior of Mg alloys. In recent years, investigations of deformation mechanisms of face-center-cubic (fcc) metals based on generalized planar fault energies have been receiving considerable attention. In this paper, we carried out first-principles calculations based on density functional theory to study the deformation behavior and mechanical properties of Al-Mg-Sc alloys. Meanwhile, the effects of RE basal stacking fault and relevant mechanical properties of Mg has also been studied and discussed. The main contents of this dissertation are as following:1. First-principles calculations have been performed to investigate the generalized planar fault energies curves of fcc Al₃Sc and Al₃Mg alloys. The results show that a three-layer thick twin may be described as the smallest twin nucleus, this is agreement with Mahajan's investigation. Twinnability of Al₃Sc and Al₃Mg has been studied by comparison. The surface energy of Al₃Sc and Al₃Mg also has been calculated, and the result show that Al₃Sc and Al₃Mg are brittle material under Rice criterion. The electronic structure is further studied in order to reveal the underlying mechanism for deformation behavior during slipping, the charge density distribution and the electronic density of states vary dramatically at the fault layers, and alters the valence-bonding hybridization, so the stacking fault of Al₃Sc and Al₃Mg is high, and dislocation nucleation would be difficult.2. The generalized stacking fault energies curves of selected Mg-X solid-solution alloys (X=Y, Sc, Nd, Eu, Er) are calculated using density functional theory. The results show that Mg-X solid-solution alloys have smaller stacking fault energies, especially the unstable stacking fault energy, and have an enhanced tendency for extended partial dislocations formation. Meanwhile, the ductility of Mg-Re alloys is fine, and the ductility of Mg-Eu alloy is relatively high compare with other Mg-Re alloys by Rice criterion.